1. Field of the Invention
The invention is directed to a method for diaphragm regulation in a computed tomography apparatus.
2. Description of the Prior Art
Computed tomography systems are known that have a measuring unit composed of a radiation source and a radiation receiver and a subject carrier arranged therebetween on which an examination subject is located. The radiation source and detector rotate another around a system axis, and, with the assistance of a computer system, the radiation transmitted to the radiation receiver is measured with respect to its intensity distribution in a detector field, which includes at least one line of radiation-sensitive image detectors arranged around the focus of the radiation source, in order to calculate the attenuation distribution in at least one plane of the examination subject. The subject occludes only a part of the fan-shaped beam from the radiation receiver. A diaphragm is disposed between the focus of the x-ray source and the subject, which is adjustable by a control arrangement to shape the radiation beam.
German OS 199 05 974 A1 and U.S. Pat. No. 5,287,396 disclose conventional X-ray devices that undertake an image evaluation, for example with contour recognition, for the diaphragm adjustment.
Computed tomography systems generate tomographic exposures of an examination subject, usually a patient, using tissue-penetrating radiation. The publication “Bildgebende Systeme für die medizinische Diagnostik,” Heinz Morneburg, Editor, 3rd Edition, particularly Chapter 5, “Prinzipien der Röntgen-Computertomographie”, describes the basic principles of computed tomography, particularly X-ray computed tomography.
In systems referred to as fan-beam devices the X-ray beam emanating from the radiation source is limited by the adjustable diaphragm to form a fan beam which penetrates the examination subject. Subsequently, the attenuation of the X-rays is measured at the other side of the patient in a detector field curved around the focus of the radiation source. The entire measuring unit composed of the radiation source and the radiation receiver rotates around the subject. The rotational axis is referred to as the system axis. The attenuation properties of the transirradiated subject in one or more planes are calculated on the basis of the measured attenuation of the radiation in the individual sectors and from different rotational angles of the measuring unit and are reproduced on a display or image carrier. Any ionizing radiation can be employed as the radiation, but X-rays generated by an X-ray tube with a rotating anode usually represents the most commonly employed radiation.
In order to achieve tomograms with good quality, it is necessary to obtain a good illumination of the detector field and retain it as much as possible during the entire exposure activity. Since the radiation stress on the patient should be as low as possible in the production of the tomographic exposure, efforts are made to limit the fan-shaped ray beam to be as narrow as possible, using an adjustable diaphragm. Such a diaphragm is disclosed, for example, in German OS 199 05 974.
Due to the narrow diaphragm setting, however, slight movements or boundary conditions that are change in some other way such as, for example, the thermal focus movement or tilting of the measuring unit (gantry tilting), frequently have a detrimental influence on the quality of the irradiation.
German Patent 42 07 006 discloses an X-ray computed tomography apparatus that has regulation of the diaphragm for these reasons. This diaphragm regulation is obtained by attaching specific position detectors at the edges of the detector field, these being struck by the fan-shaped beam and, by measuring the intensity distribution of the radiation on the specific position detectors, a migration of the beam is recognized as a consequence of a change of the focus position in the X-ray tube, so that a re-adjustment of the diaphragm can ensue immediately in order to again achieve a good irradiation of the detector field.
A disadvantage of this device and method for diaphragm adjustment is that additional, specific position detectors are needed, that are relatively complicated since the detectors must be able to identify the spatial distribution of the radiation.